DRAM memory cells over the past 15 years or more have generally been of the one transistor-one capacitor variety. Basically, charge is stored in a memory cell capacitor which is selectively coupled through the source-drain path of the transistor, sometimes called an access transistor, to a bit line. The access transistor has its gate electrode coupled to a word line. When the transistor is turned on, the charge stored in the capacitor is coupled via the source-drain path to the bit line and is generally compared against some other standard, typically a dummy memory cell, so that the state of the memory cell, indicated by the charge stored in the capacitor, is determined. For example, Heightley and Eaton U.S. Pat. No. 4,363,111 entitled "Dummy Cell Arrangement For An MOS Memory" issued Dec. 7, 1982 shows a configuration of memory cells and a dummy cell together with the sense amplifiers used in an array of DRAM cells.
Typically, each generation of DRAM has increased the memory storage capacity by a factor of four. Thus, early DRAMS with 4K of memory yielded to 16K DRAMS, then 64K DRAMS, 256K DRAMS, and 1 megabit DRAMS. With each new generation of increased memory capacity, in order to control the total area of the chip, the individual memory cells have tended to shrink in area. This has the effect of reducing the capacitance of the capacitor in the cell. In the face of this trend toward memory cell shrinkage, it has always been important to preserve sufficient capacitance to store the charge which represents the data. Currently, very large DRAMS are using trench design to increase capacitor area in a small area memory cell. The manufacturing process for this technology is extremely complex. These trenches use three dimensions to increase capacitor plate area and may be on the order of 3 microns deep and 1 micron wide. A silicon dioxide dielectric is added and then a polysilicon layer is added for a top capacitor plate.
Silicon dioxide has a fairly low dielectric constant, about 4. One approach to increasing the capacitance in DRAM memory cells is to substitute a different material as the dielectric, one that would have a higher dielectric constant than silicon dioxide has. Because capacitance is directly related to both area and dielectric constant, this would improve the capacitance for a given area.
The present invention is concerned with improving the performance of DRAM memory cells by including a dielectric other than silicon dioxide.